WITH TEMP AS (
  SELECT @S := 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
  UNION ALL SELECT @S := @S + 1 AS v1 FROM DUAL 
), OT AS (
  SELECT 1 AS idx, '2018-06-26 09:00:00' AS start_date, '2018-06-28 18:00:00' AS end_date
  UNION ALL SELECT 2 AS idx, '2018-05-26 09:00:00' AS start_date, '2018-05-28 18:00:00' AS end_date
)
SELECT
  idx,
  @T := DATE_ADD(
    DATE_FORMAT(start_date, '%Y-%m-%d %H:%i:%s'),
    INTERVAL LEVEL - 1 DAY 
  ) AS start_date,
  CONCAT(
    DATE_FORMAT(@T, '%Y-%m-%d'),
    ' ',
    DATE_FORMAT(end_date, '%H:%i:%s')
  ) AS end_date
FROM (
  SELECT
    T1.v1 + (T2.v1 - 1) * 10 AS LEVEL
  FROM
    TEMP T1,
    TEMP T2
) TEMP, OT
WHERE
  DATE_ADD(
    DATE_FORMAT(start_date, '%Y-%m-%d %H:%i:%s'),
    INTERVAL LEVEL - 1 DAY
  ) <= end_date
ORDER BY
  idx, start_date

참고로 오라클처럼 CONNECT BY LEVEL <= 형태로 가상 데이타를 만들 수 있으면 간결한데 없어서 WITH 구문으로 가상 데이타 생성하여 풀이를 했습니다.

달력 테이블을 미리 만들어 두고 조인하여 사용하시면 편리합니다.

오.. MySQL 도 WITH 구문이 되는군요.
찾아보니 분석함수(Window function) 도 지원하네요.
MySQL 8.0 / MariaDB 10.2

최신 버전이라면? 재귀쿼리도 가능하겠네요.

WITH RECURSIVE t AS
(
SELECT idx, start_date, end_date
     , start_date + INTERVAL 0 HOUR sdt
     , start_date + INTERVAL 9 HOUR edt
  FROM test4
 UNION ALL
SELECT idx, start_date, end_date
     , sdt + INTERVAL 1 DAY sdt
     , edt + INTERVAL 1 DAY edt
  FROM t
 WHERE edt < end_date
)
SELECT *
  FROM t
 ORDER BY idx, sdt
;